On the two surfaces of the body 1 of a floppy disk are formed coatings or films 2 of magnetic material which are recording media of information, as shown in FIG. 11. Such films 2 are formed in such a manner that, as shown in FIG. 12, a mixed liquid 3 of magnetic material is dropped onto the disk body 1 rotating horizontally at a high speed, at a position near the central hole 1a thereof, and the mixed liquid 3 of magnetic material is radially extended on the surface of the disk body 1 by the action of the centrifugal force applied to the disk body 1, thereby forming the film of magnetic material on the entire surface of the disk body 1.
Preferably, the whole film 2 of magnetic material has a uniform thickness so as not to cause unevenness in recordable information. However, in the conventional method, since the film is formed by the action of a centrifugal force, there is a problem in that the film thickness is larger in the section near the peripheral edge of the disk (t.sub.1 &gt;t.sub.2), and therefore, a uniform film can not be formed in the radial direction.
Hereupon, as a result of the research of the inventor of the present application, the following findings have been discovered.
As shown in FIG. 13, when the mixed liquid of magnetic material is sprayed onto the surface of the disk body 1 in the form of super fine particles using a nozzle 4 for production of super fine particles, a film of magnetic material can be formed thereon. Then, as shown in FIG. 14, an attempt was made to arrange a plurality of disk bodies 1 in series, and at the same time, place the nozzle 4 opposite the disk bodies 1, and move the nozzle 4 or the disk bodies 1, thereby forming the film 2 continuously on the disk bodies 1. Reference A indicates the region of spray, and reference L the center of the nozzle. However, with the full cone type of nozzles up to now, the density of particles per unit area is thicker in the center portion of the nozzle and is thinner in the section near the peripheral edge of the disk. Accordingly, using the method shown in FIG. 14, it was found that the coating is uniform in thickness in the direction of the X axis (the moving direction of the nozzle or disk bodies), however it is not uniform in thickness in the direction of the Y axis (the direction intersecting at a right angle to the X axis).
As a result of further research by the inventor, it was found, that with the method as shown in FIG. 13, the thickness of the film 2 formed on the surface of the disk body 1 is thicker in the section near the center portion of the nozzle and is thinner in the section near the peripheral edge of the disk, as shown in FIG. 17. Further, as shown in FIG. 18, an experiment was made by disposing the disk body 1 near the center L of the nozzle where the density of the sprayed particles is high and the distribution of the density of the particles is substantially uniform, and revolving the disk body 1 (not at a high speed, as shown in FIG. 12), and it was found that the film thickness is not uniform and is thinner in the section near the peripheral edge of the disk, as shown in FIG. 19. That is, the section near the peripheral edge of the revolving disk body 1 is greater in its peripheral speed (V=R .omega..sup.2) than the section near the center, and therefore, is lower in deposit efficiency of the super fine particles of the mixed liquid of magnetic material per unit area of the disk body 1. On the contrary, the section near the center of the disk body 1 is slower in its peripheral speed than the section of the peripheral edge, and therefore, is higher in deposit efficiency of the super fine particles per unit area of the disk body 1. This shows the fact that the film thickness becomes increasingly thinner as the position proceeds from the center to the peripheral edge, as shown in FIG. 19.
Hereupon, the inventor found that if the disk body 1 is arranged so that, in the position near the center of the nozzle where the density of fine particles in the spray region A is higher, the peripheral speed of the disk body is fast, and therefore, the deposit efficiency of the fine particles on the disk body 1 becomes lower, while in the position away from the center of the nozzle where the density of fine particles is lower, the peripheral speed of the disk body is slow, and therefore, the deposit efficiency of the fine particles on the disk body becomes higher, a uniform film thickness is obtained on the entire disk body. Based on the above-mentioned point of view, he has come to achieve the present invention.
The present invention has been made in light of the above-mentioned problems in the prior art, and the object of the invention is to provide a method and an apparatus for forming coatings on the surfaces of the disk, which enable a coating with uniform film thickness to be formed on the entire coating forming surface of the disk.